Semiconductor nanocrystals are promising materials for many important applications, ranging from bio-medical labeling, light-emitting-diodes (LEDs), lasers, solar cells, spintrinics, etc. Among these applications, many of them are taking the advantage of the size dependent photoluminescence (PL) of the nanocrystals. Up to present, the best system has been cadmium chalcogenides, especially cadmium selenide (CdSe) nanocrystals. PL of CdSe nanocrystals has reached a high yield, high stability, and broad color range (about 500-650 nm). For instance, it is possible to have a single laser as the excitation source and read 10 or more labels in the visible window with relatively stable emission and reasonable brightness using CdSe nanocrystals.
Early inventions have substantially decreased the danger associated with synthesis of high quality cadmium chalcogenide nanocrystals. The methods, known as greener approaches, are now becoming the main stream in the field of semiconductor nanocrystals. However, the apparent and intrinsic toxicity of cadmium element has attracted numerous attentions in the field, especially in the past one to two years. Unfortunately, there is no other acceptable option among the semiconductor nanocrystals. Ultimately, Si and Ge nanocrystals could be a choice. However, their indirect bulk bandgap and poorly developed synthetic chemistry cast significant doubt on them. Zinc chalcogenides, namely ZnSe, ZnS, and also ZnO, are much less toxic and could be made as high quality nanocrystals using the greener approaches. Unfortunately, their emission window may only cover purple and UV (about 450 nm and above).
Other applications, such as for solar cells and nanoelectronics, of doped semiconductor nanocrystals have been considered to be much needed. For instance, without doped nanocrystals, p-n junctions in the nanometer regime would not be possible. p-n junctions are known as the corner stone in the field of electronics and optoelectronics. At present, however, it is understood that there is no practical method to make p- or n-doped semiconductor nanocrystals.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.